1. Field of the Invention
This invention relates to a drum brake device with an automatic shoe clearance adjustment apparatus, and, more particularly, to a shoe clearance over-adjustment prevention means which suspends automatic shoe clearance adjustment when the temperature within the brake increases and exceeds a predetermined range.
2. Description of Related Art
Generally, a drum brake has an automatic shoe clearance adjustment apparatus designed to compensate wear of the brake shoe lining in order to maintain a certain shoe-to-drum clearance.
If the drum brake is repeatedly applied, for example, if a vehicle is driving down a long downward slope, the temperature of the brake drum increases and thereby expands the brake drum to cause an apparent shoe-to-drum clearance enlargement, i.e., a temporary enlargement of the shoe-to-drum clearance to the extent of brake drum expansion.
While the drum brake is expanding, if the automatic shoe clearance adjustment apparatus is configured such that an automatic shoe clearance adjustment functions to adjust the shoe-to-drum clearance, temporarily enlarged because of the brake drum expansion, then when the brake drum is cooled and the temperature thereof decreases, the brake drum shrinks and causes it to return to its normal state, the shoe-to-drum clearance already adjusted during the temporary brake drum expansion remains the same and this can be considered an over-adjustment and may possibly cause a dragging problem.
If the shoe-to-drum clearance in a normal state (not in a use under a predetermined high-temperature) is preset to be wider in order to avoid dragging, a shoe clearance over-adjustment could be prevented, but strokes, such as a brake pedal stroke and a brake lever stroke, become larger and this then causes a delay in the braking effect and gives the driver a disconcerting brake feeling.
As an example of technology to resolve the above problems, for example, the Japanese Patent Publication No. 58-49739 discloses a drum brake with a shoe clearance over-adjustment prevention means.
An incremental type automatic shoe clearance adjustment apparatus employed in the drum brake of this conventional art is equipped with a shoe clearance adjustment strut which is composed of two, axially detachable members (e.g., a tube and a nut) and a bimetal strip, a component of a shoe clearance over-adjustment prevention means, placed therebetween.
In the structure disclosed in the above-conventional art, the shoe clearance over-adjustment means cannot be used in most other types of automatic shoe clearance adjustment apparatus, such as one-shot type. It may, however, be used in the incremental type.
Applicant of this invention filed the Japanese Patent Application No. 2000-297651 (Provisional Patent Publication JP-2002-106620-A equivalent to US-2000-0036123-A).
A shoe clearance over-adjustment prevention means of the drum brake in this second conventional art provides a thermo-sensitive member, either between a shoe clearance adjustment strut and a brake shoe, or between the shoe clearance adjustment strut and a brake lever.
The shoe clearance over-adjustment prevention means in the structure of the second conventional art can be employed in a one-shot type automatic shoe clearance adjustment apparatus. Therefore, the second conventional art provides excellent applicability in this regard.
A typical one-shot type automatic shoe clearance adjustment apparatus comprises a shoe clearance adjustment strut having a strut body, a bell crank adjustment lever, a pin pivoting the adjustment lever on the strut body, a first spring extended between the strut body and first brake shoe and a second spring. If the amount of outward movement of a pair of brake shoes during the wheel cylinder operation exceeds a predetermined distance, the strut body follows the first brake shoe and a brake lever because of the force of the first spring, and the second brake shoe acts on the adjustment lever to move an engagement position between the adjustment lever and the strut body so as to extend the entire length of the shoe clearance adjustment strut, thereby automatically adjusting the shoe-to-drum clearance.
The shoe clearance over-adjustment prevention means is characterized in that a thermo-sensitive member is folded thereby creating a folded section and two legs extending therefrom to be folded and facing each other, forming a superposed portion, is inserted between the strut body and the brake lever and that-when the service brake operates and the temperature in the brake drum increases and the temperature of the thermo-sensitive member exceeds the predetermined range, the superposed portion of the thermo-sensitive member opens and moves outward to spread the two legs apart from each other against a force of the first spring so as to move the shoe clearance adjustment strut to follow the second brake shoe suspending the automatic shoe clearance adjustment, thereby preventing the over-adjustment.
In the above-described shoe clearance over-adjustment prevention means, the thermo-sensitive member requires the force be more than the spring force of the first spring because the superposed portion of the thermo-sensitive member is positioned close to the first spring. Therefore, the thermo-sensitive member needs to be made from a thicker material and must have a wider design.
Accordingly, weight (or mass) of the thermo-sensitive member inevitably increases, thereby increasing the cost thereof.